The present invention relates to a diagnosis system for internal combustion engine mounted on a motor vehicle.
An electronic fuel-injection system of the type which is provided with a computer operable to compute various input signals from sensing devices, such as a throttle position sensor, intake airflow meter, intake manifold vacuum sensor and coolant temperature sensor in order to produce output signals for driving solenoid-operated fuel injectors is known. For example, the coolant temperature sensor is provided to produce an output signal, when the temperature elevates to a predetermind value. The computor uses the output signal as an input signal representing engine warming-up. However, if fault, such as disconnection of wires for the sensor or short circuit of the sensor occurs in the coolant temperature sensor system, the sensor produces an output having extreme value, which results in failure of the controlling of air-fuel ratio of injected fuel and in stalling of the engine.
In order to avoid such difficulties, a fail-safe system is proposed. The fail-safe system is so arranged as to produce a fail-safe signal, when fault in a sensing device continues for a predetermined time and to produce a quasi signal which causes fuel injectors to supply a proper amount of fuel to the engine to maintain the operation of the engine. At the same time, the fail-safe signal causes a driver circuit to light a warning lamp indicating the fault. However, it is undesirable to a driver of the vehicle to warn such a fault which recovers after a very short time. On the contrary, fail-safe operation should be started as soon as possible, in order to prevent stalling of the engine.